The present invention relates to a projecting lens used for enlarging an image appearing on a relatively small CRT (cathode-ray tube) or the like so as to project it onto a screen, and to a projection-type television in which this projecting lens is mounted.
It is a well-known technology that, in order to obtain an image appearing on a TV screen, this TV image which is displayed on a relatively small CRT, and is then enlarged and projected by projecting lens onto wider screen. A system has been employed recently in which a space between a projecting lens and a CRT is filled with a liquid. This liquid has a high transparency and a refractive index close to that of the surface glass pane of the CRT. The projecting lens and the CRT are then optically coupled together. This optical coupling considerably reduces interfacial reflection, thus efficiently transmitting light (an image) radiated from the CRT to the projecting lens. The clarity and contrast of a projected image are thus improved. Such a system is generally referred to as an optical coupling system.
FIG. 1 shows a conventional projecting lens optical unit. Numeral 1 denotes projecting lenses, and numeral 2 denotes an optical coupling (OC) housing. A CRT 3 on which an image is displayed is attached to the right side of the OC housing 2, and an OC lens 4 is attached to the left side of the OC housing 2. This OC lens 4 is positioned on the extreme right of lens elements which constitute the projecting lenses 1. Sealing rubber grooves 5 and 6 are formed in the OC housing 2. Sealing rubbers 7 and 8 are inserted into the sealing rubber grooves 5 and 6, respectively. When the OC lens 4 is attached to the left side of the OC housing 2 with the CRT 3 being attached to the right side of the OC housing 2, the sealing rubbers 7 and 8 are pressed and deformed. The sealing rubbers 7 and 8 are thereby closely press-contact with the OC lens 4 and the CRT 3. The CRT 3 and the OC lens 4 are thus sealed so that a liquid can be contained in a watertight condition. An optical coupling liquid (hereinafter referred to as an OC liquid) 9 is filled into the space enclosed by the OC housing 2, the CRT 3 and the OC lens 4. A filling hole 10 is sealed so that the OC liquid 9 will not leak. At present ethylene glycol, a liquid mixed with ethylene glycol and glycerin, or a liquid mixed with ethylene glycol and water is generally and often used as the OC liquid 9.
As soon as the CRT 3 is operated, an image displayed on a raster screen is enlarged and projected onto a screen (not shown) installed in front of the projecting lenses 1. In a projecting lens unit as constructed in FIG. 1, the OC liquid 9, like the other components of the unit, functions as an important factor in the optical system. A variation in the refractive index of the OC liquid 9 therefore greatly affects the optical properties.
Because the CRT of a projection-type television must output a great amount of light to enlarge and project images, it requires much more electricitY than that used for operating the CRT of a direct viewing tube type television. For this reason, the temperature of the CRT itself, particularly the surface temperature of a raster portion, increases to 100.degree. C. or more. The OC liquid 9 mentioned above also functions as a cooling liquid. Heat generated by operating the CRT is transmitted through the OC liquid 9 to the OC housing 2, where it is dissipated.
As the temperature of the OC liquid 9 increases, its refractive index decreases. The refractive index (Ne line) of, for example, ethylene glycol, is 1.4308 at 20.degree. C., but falls to 1.4146 at 80.degree. C.
As the CRT 3 is operated, the OC liquid temperature gradually increases, reaching an equilibrium at about 80.degree. C. Because the refractive index of the OC liquid 9 varies, even when the focus is precisely adjusted at first at about 20.degree. C., it deviates, as the OC liquid temperature increases. Therefore a projected image becomes out of focus.
As has been described above, when the CRT 3 is operated, the OC liquid temperature inevitably increases, thus causing its refractive index to decrease. For this reason, the optimum focus screen deviates from a position where it is initially positioned, and the projected image thereby becomes out of focus. The conventional art does not cope with such a problem.
The present invention has been made in order to solve the above-mentioned problem. The object of the invention therefore is to provide a simply-constructed projecting lens unit in which a focus screen can be corrected in an effective and rational manner.